Session Z17: Astrophysics with Gravitational Waves II

Live

We study the merger rates of binary black holes in globular clusters that are among the most dense stellar environments and a natural place for the creation of black hole binaries. To model these systems with all their variations we rely on the observational properties of the known Milky Way globular clusters. We include interactions of black hole binaries with stars, with compact objects of mass between 2.5 and 5 solar masses (low mass gap objects) and with black holes as third bodies. Especially the soft interactions with stars accelerate the evolution of these binaries. We implement a simple model to evaluate the merger rate of black hole-low mass gap objects mergers formed in globular cluster-type environments. We start with star-star binaries and through successive binary-single exchanges end creating black hole binaries, black hole-low mass gap pairs and double low mass gap binaries. We monitor both the different populations of single compact objects, as well as the different combinations of star/low mass gap/black hole binary populations. We find that under proper conditions that exist in certain globular cluster environments, black hole-low mass gap mergers can take place at a rate suggested by the detection of GW190814 observed by LIGO's O3 run.   

Live

Future gravitational-wave (GW) detector networks will detect binary-black-hole mergers (BBHs) up to redshift of z~20, allowing for a direct measurement of the merger history in the “early” universe. Here we show how GW detections of BBHs made by proposed third-generation detectors can be used to search for the Population III (Pop III) binaries dominating at redshift z~10. To identify these Pop III binaries, we first perform a mock data analysis based on the redshift distribution predicted by cosmological simulation, and infer its characteristic peak using Gaussian process regression. Then, we employ phenomenological models to further extract the branching ratio between the Pop III BBHs and the Pop I/II BBHs, as well as the characteristic shape parameters of the subpopulations. With one month of observations at the predicted rate of ~200 Pop III BBHs per month, parameters describing its volumetric merger rate can be constrained at the O(10%) level.   

Live

In this talk I will describe a novel framework to characterize the population of binary black holes using detections of arbitrary significance. I will quantify the information gain from the inclusion of marginal events and introduce a theoretical bound on the information content of the astrophysical stochastic background, derived with this framework. I will report constraints on the distributions of merging binary black hole masses, spins and rate derived from detections from the first two LIGO-Virgo observing runs, including those identified by our group, and how these get updated with results from the recent third observing run.   

Live

The mass and spin properties of black hole binaries inferred from their gravitational-wave signatures reveal important clues about how these binaries form. For instance, stellar-mass black holes that evolved together from the same binary star will have spins that are preferentially aligned with their orbital angular momentum. Alternatively, if the black holes formed separately from each other and later became gravitationally bound, then there is no such preference for having aligned spins. Furthermore, it is known that the presence of misaligned spins induces a general relativistic precession of the orbital plane, imprinting unique structure onto the gravitational-wave signal. The fidelity with which gravitational-wave detectors can measure off-axis spins, or equivalently, precession, will therefore have important implications for the use of gravitational waves to study binary black hole formation channels. I will summarize a new study that examines how well the A+ detector network will measure off-axis spin components, and report preliminary results comparing spin resolution between the fourth and fifth LIGO-Virgo observing runs using simulated detector noise and multiple sets of simulated signals distributed over the mass-spin parameter space.   

Not Participating

When two black holes merge, loss of linear momentum through gravitational radiation can impart a recoil velocity, or a “kick”, to the final black hole. While previous studies have shown that it is difficult to constrain the kick of individual gravitational-wave events at current detector sensitivities, it may still be possible to extract information about the kicks of the binary population as a whole. In this work, we model the kick for each event as drawn from a common underlying distribution, whose properties we infer from the data. We place constraints on the allowed distributions of kicks, which can be used to predict remnant black hole retention rates, and therefore constrain rates of second-generation mergers in different formation environments.   

Live

Now that we are in a new era of gravitational wave detection it is worth asking how far we can take these measurements in searching for new physics. But to do so we also want to better understand the signals we do see. One outstanding question is the origin of binary black holes. By studying the effects of binary eccentricity on measurements at current and future gravitational wave detectors, we can shed light on the environment in which the black holes were created. We also argue for other interesting measures of the black hole environment and discuss how future gravity wave experiments could probe dark matter or physics associated with the electroweak scale.   

Live

Current gravitational-wave data analysis of merging binary black holes accounts for two precessing spins, allowing inference of the six spin degrees of freedom. Nonetheless, it is convenient to use effective parameters to interpret detections; the effective aligned spin $\chi_{\rm{eff}}$ and precessing spin $\chi_{\rm{p}}$ measure spins parallel and perpendicular to the orbital angular momentum, with measurements away from zero indicating large spins and significant precession, respectively. While $\chi_{\rm{eff}}$ is conserved during an inspiral, $\chi_{\rm{p}}$ is not; furthermore, it employs a single-spin approximation that retains some, but not all, precession-timescale variations. To rectify this inconsistency we propose two-spin definitions that either fully consider or fully average those variations. In addition to the previous domain $\chi_{\rm{p}}\in[0,1]$, our generalized parameter presents an exclusive region $1\leq\chi_{\rm{p}}\leq2$ accessible only to binaries with two precessing spins. For current LIGO/Virgo events, our generalized parameter indicates that, while (i) previous measurement errors on $\chi_{\rm{p}}$ may be underestimated, (ii) the evidence for spin precession may be stronger than suggested previously.   

Live

Gravitational-wave (GW) memory effects are lasting changes in the GW strain and its time integrals following bursts of GWs. They are closely related to the symmetries of asymptotically flat spacetimes and their corresponding conserved charges. There are three types of GW memory effects (displacement, spin, and center-of-mass) that are related to different conserved charges and have different observable effects. GW memory effects are well studied in general relativity (GR) but have not been explored as carefully in theories beyond GR. One of the simplest modified theories of gravity is Brans-Dicke theory, which includes a massless scalar field nonminimally coupled to gravity. This theory has a scalar breathing polarization of GWs in addition to the tensor GWs in GR, and there can be scalar GW memory effects in addition to the tensor GW memory effects. The scalar memory effects are not related to symmetries or conserved quantities, but the scalar waves (and their memory) do affect the tensor memories. I will present the leading Newtonian corrections to the tensor displacement and spin GW memory effects from nonspinning, quasi-circular compact binaries in Brans-Dicke theory.   

Live

The gravitational wave memory effect reflects a permanent change in separation of a pair of initially comoving test particles caused by a burst of gravitational waves. Near null infinity, the contributions to the memory effect split into two categories: linear memory, which appears even in linearized gravity, and nonlinear memory, which arises due to the nonlinear nature of general relativity. Moreover, this nonlinear effect is expected to be the dominant contribution to the memory effect for binary black hole mergers, such as those detected by LIGO and Virgo. In this talk, we discuss how the final separation of test particles depends not only on initial separation (as in the usual memory effect), but also the initial relative velocity and the relative acceleration of the test particles. Each of these contributions provides additional memory-like effects near null infinity, and we show that, like the usual memory effect, a similar linear vs. nonlinear split arises.